In the promotion of many paper or textile products such as personal care products, the perceived texture and softness of the product by the consumer is important for its general acceptance and use. To obtain this desired texture and softness, a condition that is usually loosely defined and difficult to describe, manufacturers expend considerable time and effort adjusting their manufacturing process to produce materials with a fiber content, size, and dispersion that ultimately "feels soft" in the final product. Often, the determination of texture and softness is accomplished by a team of human evaluators who touch and manipulate product samples to evaluate the quality and acceptability of the material. While the judgment is mostly qualitative, this typical approach to quality assurance has generally worked well in the past.
Unfortunately, the use of human evaluators has a number of limitations. The decision of whether or not a material has an acceptable texture and softness is largely qualitative, and the number of process variables involved in achieving this softness is large. Often it is difficult, without considerable analysis, to precisely locate operations within the manufacturing process that may require adjustment. Furthermore, because human beings are subject to sickness, emotional stress, and dermatological ailments, among others, the judgment of texture or softness made by the evaluation team is not always accurate and repeatable. This fluctuation in judgment can be detrimental to the manufacturer in two ways:
(1) If good material is judged as unacceptable, considerable time and resources are needlessly wasted in trying to correct the situation.
(2) More importantly, if poor material is judged as acceptable, the product may eventually be rejected by the consumer, adversely affecting the manufacturer's reputation.
In response to the need for more reliable quality assurance testing, texture sensors have been incorporated into the evaluation process. While not intended to replace the human evaluators, these devices are designed to assist in the human judgment and provide a more consistent and quantitative approach to quality control. The most typical approach used in conventional texture sensors consists of a stylus (similar to that used in a phonograph), a displacement transducer, and signal processing circuitry. Such a configuration yields an indication of fiber size, dispersion, and composition as the tip of the sensor is pulled across the surface of the material. Like the phonograph needle on a record, the texture sensing stylus is deflected by the topography of the material, and the output signal is processed to determine the perceived surface softness of the material.
While the concepts of a texture sensing stylus works well in principle, its actual implementation has a number of shortcomings when actually applied to textiles and paper products. The test can be difficult to set up and execute because of irregularities in the product being tested. The rigid stylus can get snagged on fibers protruding above the surface and tear the material, negating any useful results. In other instances, only a qualitative assessment of the material's softness can be made.
The primary problem associated with using a rigid stylus on tissue-type materials is that the stylus is not well-suited for such an application. Rigid styluses have routinely been used to measure surface roughness of machined metals with great success. The sharpness and stiffness characteristics of the stylus give it its sensitivity and accuracy to measure machined surface roughness to a resolution on the order of microinches. Unfortunately, these same qualities are its downfall when applied to textile and paper products. Fibers extending from the surface can catch on the sharp stylus as it progresses across the surface of the material being measured. Other flexible fibers comprising the material can be torn or moved out of the path of the stylus, resulting in a trough through the material surface that traces the stylus' path, as well as resulting in a poor measurement of surface features.
Attempts to modify the stylus approach for use on personal care products have produced limited results at best. In order to prevent the device from destroying the surface of the material, sensitivity to surface topography measurement is often sacrificed.
Because the results are not always reliable, the stylus sensor has not gained widespread acceptance in texture sensing, and, despite its limitations, the use of human evaluators is often considered more cost effective. Therefore, the need for an accurate, reliable, and cost-effective approach to surface texture sensing still exists.
Alternatives to the rigid stylus/vibration sensor arrangement for quickly and accurately determining the surface quality of personal care products have been generally unavailable. However, advances in sensing technologies and signal processing offer a solution to this problem.